Correction system for continuous rolling mill

ABSTRACT

In a tandem mill a roll current for a first mill stand is measured after a workpiece has entered only the first stand and after it has also entered a second stand. A difference between the measured currents predicts a speed correction factor for the second stand by which the speed of the second stand is modified. Then the roll current for the first stand is again measured and the speed of the second stand is adjusted so as to put a difference between the first and third measured current within predetermined limits. Otherwise, the above process is repeated until a difference between the first and last measured currents is put in the limits. The last speed correction factor determined is stored in a computer.

United States Patent 1151 3,695,075 Kubota 1 Oct. 3, 1972 [54]CORRECTION SYSTEM FOR 3,540,248 11/1970 Hostetter et al. ..72/8

CONTINUOUS ROLLING MILL Primary Examiner-Milton S. Mehr [72] InventorNobuo Kubota Kobe Japan Attomey-Robert E. Burns and Emmanuel J. Lobato[73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [57] ABSTRACT [22] Filed: June 9, 1971 In a tandem mill :1roll current for a first mill stand is measured after a workpiece hasentered only the first [21] Appl 15l442 stand and after it has alsoentered a second stand. A difference between the measured currentspredicts a [30] Foreign Application Priority Data speed correctionfactor for the second stand by which the speed of the second stand ismodified. Then the June 11, 1970 Japan ..45/50439 roll current for thefirst stand is again measured and the speed of the second stand isadjusted so as to put a [52] US. ((51 difference between the first andthird measured cup [51] Int.Id 72/8 17 6 rent within predetermined mimOtherwise, the above [581 Fla 0 process is repeated until a differencebetween the first R f C1 d and last measured currents is put in thelimits. The last [56] e erences I e speed correction factor determinedis stored in a com- UNITED STATES PATENTS P 3,457,747 7/1969 Yeomans..72/19 2 Claims, 11 Drawing Figures #2: l M 5 K rukmn Z rn/vm. f

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PATENIEHUCIB I972 SHEET 8 0F 8 CORRECTION SYSTEM FOR CONTINUOUS ROLLINGMILL BACKGROUND OF THE INVENTION This invention relates to a workpiecethickness control system for a continuous rolling mill and moreparticularly to a workpiece thickness control system for a multiplestand tandem mill by which workpieces can be continuously rolled in theabsence of a tension or a pressure exerted upon that portions of theworkpieces traveling between each pair of adjacent mill stands.

Upon continuously rolling a workpiece by a multiple stand tandem rollingmill, the workpiece is rolled while it bridges rolling mill stands. Inorder that the rolled products are particularly increased in accuracy ofdimension, it is required to roll workpieces while each pair of adjacentmill stands do not exert any tension or pressure upon that portion ofthe workpiece traveling therebetween.

This purpose has been previously accomplished by the following twoprocesses: One of the processes has been to roll a workpiece while aloop is formed along the workpiece traveling path between each pair ofadjacent mill stands as in hot strip mills, wire mills, etc., while thatprocess is possible to be effected with workpieces small in dimension,it is difficult to form such loops with large workpieces because theyincrease in V bending stress with an increase in dimension thereof.

Also the loop may have preferably a slack as small as possible but thismeasure has encountered a difficulty in stably controlling the speeds ofrolling mill stands.

The other process has been to detect a tension or a pressure exerted onthat portion of a workpiece traveling between each pair of adjacent millstands to control the speeds of the mill stands. This detection of thetension or pressure is generally accomplished by measuring the rollcurrent. However the roll current greatly depends upon various factorsaffecting the system operation, rendering the control of the speeds ofthe mill stand difficult.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention toprovide a new and improved workpiece thickness control system for amultiple stand tandem rolling mill capable of continuously rollingworkpieces with no tension nor a pressure exerted on the workpiecetraveling between rolling mill stands in the rolling mill and in whichthe disadvantages of the prior art practice as above described areeliminated.

The invention accomplishes this object by the provision of a workpiecethickness control system for a multiple stand tandem rolling millcomprising at least two rolling mill stands, wherein there are providedmeans for measuring a roll current for a first rolling mill stand, meansfor determining a variation in the roll current means for predicting acompensation component of a speed of a second rolling mill stand fromthe variation in the roll current, to modify the speed of the secondrolling mill stand and means for controlling the speed of the secondmill stand so as to put a variation in a roll current again measured forthe first rolling mill stand within predetermined limits.

Preferably the workpiece thickness control system may comprise means forstoring roll current for a first rolling mill stand, means for detectinga current difference between the stored roll current for the firstrolling mill stand and a roll current for a second rolling mill standobtained after a workpiece enters the second rolling mill stand andmeans for controlling an oscillator forming a sampling switch so as toput the current difference within predetermined limits thereby to adjustthe speed of the second rolling mill stand.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readilyapparent from the following detailed description taken in conjunctionwith the accompanying drawings in which:

FIG. 1 a schematic view of a detection mechanism for use with theconventional type of workpiece thickness control systems for continuousmetal strip rolling mills to detect a tension or a pressure on a metalstrip traveling between a pair of adjacent rolling mill stands;

FIG. 2a is a schematic view of a plurality of rolling mill stands of atandem stand roughing wire mill;

FIGS. 2b and c are graphs illustrating rolling currents for therespective rolling mill stands shown in FIG. 2a under the control of aconventional tension-less control system;

FIG. 3 is a combined block and circuit diagram of a speed control systemof the conventional type operatively coupled to a rolling mill stand ofa blooming mill with parts illustrated in perspective;

FIGS. 4 and 5 are views similar to FIG. 3 but illustrating modificationsof the arrangement shown in FIG. 3;

FIG. 6 is a block diagram of a workpiece thickness control system foruse with multiple stand tandem rolling mills constructed in accordancewith the principles of the invention;

FIG. 7 is a block diagram of a modification of the invention;

FIG. 8a is a schematic view of the rolling mill stands shown in FIG. 7;

FIGS. 8b and c are graphs illustrating roll currents for the respectiverolling mill stands plotted against time;

FIG. 8d is a curve plotting a stand speed against time; and

FIG. 9a, b and c are logic flow charts explaining the operation of thearrangement shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of thedrawings, there are illustrated a first and a second rolling mill standSI and SH respectively each including a pair of work rolls 10' and 12between which a workpiece 14 is to be reduced in area. The rolling millstands SI and SI] are operatively coupled to individual press-ductors 16used to detect a tension on that portion of the workpiece 14 extendingbetween both mill stands. Then controls not shown are used to controlthe speeds of the rolling mill stands so as to exert a null tension orpressure upon the workpiece. The arrangement of FIG. 1 is generally usedwith the conventional workpiece thickness control systems for multiplestand tandem rolling mills.

However, in small-sized and moderate sized rolling mills, rolling millstands are generally exchanged for new ones after some time intervals ofservice leading to a disadvantage in view of the stand point of themaintenance of press-ductors disposed on the mill stands.

Therefore the exchange of rolling mill stands is of no utility also ithas been already proposed to measure a roll reaction on a rolling millstand to give a measure of a roll force exerted by the mill stand.However there have not yet been developed the equations describing therelationship between the roll reaction and roll force on the rollingmill stand with a sufficiently high accuracy.

Referring now to FIG. 2a a workpiece 14 has been already rolled byopposed work rolls l and 12 on a first and a second rolling mill standsSI and SH and reached somewhere short of a third rolling mill stand SIIIof a roughing wire mill. Upon entering the workpiece 14 into the firstmill stand SI a current flows through a driving motor (not shown)operatively coupled to that stand as shown at curve I in FIG. 2b. Thenthe workpiece 14 enters the second mill stand SII whereupon anotherdriving motor (not shown) operatively coupled to that stand has flowingtherethrough a roll current I, as shown in FIG. 2c. At the same time theroll current I is temporarity decreased then the mill stand SII isfinely adjusted in speed to control the roll current I for the firstmill stand SI to its original steady-state magnitude whereby the tensionis removed from that portion of the workpiece l4 traveling between themill-stands SI and SII. The process as above described is repeated withthe succeeding mill stands such as a stand SIII to effect thetension-less workpiece thickness control.

Referring now to FIG. 3, there is illustrated a speed control system ofconventional construction for use with blooming mills for one rollingmill stand thereof. The arrangement illustrated comprises a pair ofvertical work rolls and 22, a drive motor 24 connected to drive thevertical rolls 20 and 22, and a pair of horizontal work rolls l0 and 12driven by another drive motor (not shown) having a speed controlled inaccordance with a speed compensated for screwdown by the vertical rolls20 and 22. A load cell 26 is disposed below the horizontal work rolls l0and 12 to sense the roll separation force between the rolls 20 and 22.That is, the load cell 26 is responsive to a workpiece 14 being enteredbetween the horizontal rolls 20 and 22 to supply a correspondingelectric signal to a metal-in-stand circuit 28 connected to groundthrough an operating winding of a transfer relay 30.

The drive motor 24 is connected across a source of electric power 32through a current transformer 34. The source is controlled by thyristorsto control the speed of the motor 24 and the current transformer 34 isoperative to detect a current flowing through the drive motor 24 and tosupply an output to a current detector 36. The transfer relay 30includes two sets of normally open contacts 30a and b and two sets ofnormally closed contacts 30c and d. The current detector 36 is coupledthrough the normally closed relay contacts 30d to an operationalamplifier 38 having connected between its output and input a capacitorand a series combination of a resistor and the normally closed contacts300. The operational amplifier 38 is connected at the output to anotheroperational amplifier 40 through the normally open relay contacts 30aand a resistor while the current detector 36 is also connected directlyto the operational amplifier 40 through the normally open contacts 30band a resistor. The amplifier 40 has a feedback resistor and isconnected to a current control circuit 42 subsequently connected to avoltage control circuit 44. Then the voltage control circuit 44 isconnected to the source 32.

The arrangement thus far described is operated as follows: When theworkpiece enters between the vertical rolls 20 and 22, a current flowsthrough a circuit with the roll motor 24 in accordance with theparticular roll loading. That current is detected by the currentdetector 36 through the current transformer 34 and the detected currentis applied to the operational amplifier 38. The operational amplifier 36is pre-set to provide an output voltage equal to that from the currentdetector 36.

Then the workpiece 14 will be entered between the horizontal work rollsl0 and 12. At that time the load cell 26 supplies an output to themetal-in-stand circuit 28 indicating that the workpiece has been enteredbetween the horizontal rolls l0 and 12. This causes the energization ofthe relay winding 30w to open the contacts 30c and d whereupon theoperational amplifier 38 changes the operation from the time delay modeto the integration mode. This means that the operational amplifier 38holds that magnitude of the output current developed at the instant therelay 30 has been just energized. That magnitude of the output currentis applied through the now closed contacts 30a to the operationalamplifier 40, also having applied thereto the actual magnitude of thecurrent flowing the motor 24 through the now closed contacts 30b.

Therefore the operational amplifier 40 provides an output current equalto a difference between the roll current for the vertical work rolls 20and 22 stored by the operational amplifier 38 and the current detectedby the current detector 36 and applied to the amplifier 40. This outputcurrent or current difference is delivered to the current controlcircuit 42. The current control circuit 42 is operative to control thesource 32 through the voltage control circuit 44 and by means of thethyristors disposed in the source 32 until the current differencebecomes null. In other words, the control is effected to equal thecurrent controlling the vertical rolls 20 and 22 to the current storedby the operational amplifier 38.

The process as above described is effected on the basis of the currentcontrol in which the speed of one set of work rolls is rendered equal tothat of the other set of work rolls through the intermediary of thecorresponding currents. Therefore the arrangement is disadvantageous inthat any acceleration or deceleration directly reflects a variation inforce exerted on a workpiece between the adjacent roll stands and thatone must take into account of influences in the system operationresulting from a decrease in temperature of a workpiece being rolled, achange in loading such as skied marks on the workpiece and so on.

FIG. 4 wherein like reference numerals designate the componentsidentical or similar to those shown in FIG.

3 illustrates a modification of the arrangement as shown in FIG. 3. FIG.4 shows a pair of succeeding rolling mill stands SI and SII of amultiple stand tandem blooming mill. One drive motor 24-l or -II foreach mill stand is adapted to drive a pair of horizontal work rolls NHand l2I or 10 and l2II rather than the vertical work rolls such as shownin FIG. 3 and operatively coupled to an individual pilot generator 461or 4611 connected to the respective thyristor controlled source 32- I or-lI through an amplifier and also to a variable resistor 481 or 48H forsetting up the speed of the associated mill stand SI or SII. Then thevariable resistor 48-1 and -II are connected to a positive buss B+.

For each stand, the metal-in-stand circuit 281 or 28II is connected toground through an individual relay 30I or 3011 including two sets ofnormally open contacts 30Ia and b or 3011a and b. By omitting thecurrent detector 36 as shown in FIG. 3, the current transformer 32 isdirectly coupled to the operational amplifier 38 through the relaycontacts 301!) and also to the operational amplifier 40 through therelay contacts 30IIb with the relay contacts 301a and 30Ila substitutedfor the relay contacts 300 and 30a shown in FIG. 3. The operationalamplifier 40 is connected to the junction of the pilot generator 46IIand the setting up resistor 48H. In other respects the arrangement isidentical to that shown in FIG. 3.

After the setting up of the starting conditions including the magnitudeof the resistors 48I and 48II, both the motors 241 and 24Il areenergized to start the respective mill stands SI and SII. The workpiecel4 enters the first mill stand S] to permit the load cell 261 toenergize the relay 301 through the metal-in-stand circuit 281. Whenenergized, the relay 30I closes its contacts 301a and b to cause acurrent flowing through the motor 24I to be stored by the operationalamplifier 38 through the current transformer 34.

Similarly, the workpiece entering the second mill stand SII causes theenergization of the associated relay 30Il. Therefore its contact 30Ilaand b are closed to deliver to the operational amplifier 40 both theroll current for the first mill stand SI stored by the operationalamplifier 38 and the actual roll current for the same mill stand. Acurrent difference between these roll current provided by theoperational amplifier 40 is applied to the junction of the pilot motor4611 and the resistor 48Il until it is corrected.

This results in the workpiece thickness control being continuouslyeffected under non-tensioned state.

After the workpiece 14 has passed through the arrangement as shown inFIG. 3, the speed setting up resistor 4811 can be set to the commandmagnitude corresponding to the corrected speed obtained with thatworkpiece. This measure permits head end gauge of or gauges of thesucceeding similar workpiece or workpieces to increase in accuracybecause the mill stands have now their speed more proper than those setfor the previous workpiece.

With a third mill stand (not shown) following the second mill, it is tobe noted that the speed correction on the second mill stand should havebeen acomplished before the workpiece will enter the third mill stand.

In the process as above described, the tensions or pressures due to thefirst and second mill stands serve to effect the closed loop controlthrough both the roll current for the first mill stand and the speed ofthe second mill stand. However the roll current greatly depends upon thematerial, temperature and cross sectional area of a workpiece beingrolled, percent rolling reduction, the magnitude of the motor's fieldexcitation, etc. This results in great changes in factors affecting thesystem operation. Among them that factor affecting the roll current isparticularly large in change as will be readily understood from theequation where I is a current flowing through a rolling motor for a millstand, A! is a variation in the current I, A is a control voltage forcorrecting the speed of the succeeding stand, AA is a'variation in thevoltage A and K is a constant. Therefore it has been difficult to form astable control system of the arrangement as shown in FIG. 4.

Therefore it can be presumed that the relationship between a variationin roll current and a speed of a rolling mill stand may be preferable tobe somewhat loose. To this end, a difference signal for a roll currentfor the first mill stand may be used to drive a servooperatedpotentiometer thereby to control the speed of the second mill stand.Alternatively an on-off switch may be operated to control sampling whileat the same time a sensitivity switch is used to compensate for thesystem gain.

FIG. 5 show a tension-less workpiece thickness control system includinga servo-operated potentiometer such as above described. In FIG. 5 wherelike reference numerals designate the components identical to thoseshown in FIG. 4, the operational amplifier 40 is connected to aservo-motor 49 for controlling the speed setting up resistor 48H but notconnected to the junction of that resistor and the pilot generator 46.In other respect, the arrangement is identical to that shown in FIG. 4.The arrangement is disadvantageous in that the system response is slow.Assuming that the law of conservation of mass flow is held, it isrequired only to maintain a constant value of a speed ratio between thesuccessive mill stands.

Referring now to FIG. 6, there is schematically illustrated a workpiecethickness control system for a multiple stand tandem rolling millconstructed in accordance with the principles of the invention andparticularly operative in the'sampling mode. The arrangement illustratedcomprises an input terminal 50, leading to a current detector for rollcurrent such as labelled 36 in FIG. 3, a first operational amplifier 52acting as an integrator and a second operational amplifier 54 connectedin the similar manner as above described in conjunction with theoperational amplifiers 38 and 40 as shown in FIG. 3. More specifically,the input terminal 50 is coupled through a set of normal closed contactsRlb to the operational amplifier 52 including a capacitor, a resistorand a set of normal closed contacts Rla in its feedback circuit andserving to store a roll current under non-tensioned state for theassociated mill stand, for example, a first mill stand of a multiplestand tandem rolling mill (not shown), as previously described inconjunction with FIG. 3. The operational amplifier 52 is coupled througha set of normally open contacts R2d to the operational amplifier 54including a capacitor, a resistor and a set of normally open contactsMe. The operational amplifier 54 is then connected to a tapped resistor56 including a plurality of switching taps, in this case, 10 taps 1through 10 selectively coupled to an operational amplifier 58 through aset of normally closed contacts R2b. The operational amplifier 58includes a resistor, a capacitor serially interconnected and connectedacross a set of normally closed contacts R3a in its feedback circuit andforms a proportion plus ingration circuit having a proportional constantand an integration constant sufficient to limit an overshooting of aroll current for the succeeding or second mill stand due to the risingportion of the output therefrom, to a magnitude as low aspossible. Theoperational amplifier 58 is connected to an output terminal 60 leadingto an input to a speed control limiter (not shown).

The input terminal 50 is further coupled through a set of normally opencontacts R2e to the operational amplifier 54 and through a set ofnormally open contacts Rld to a comparison amplifier 62 including afeedback resistor with the integrator 52 coupled to the comparisonamplifier 62 through a set of normally open contacts Rlc. The comparisonamplifier 62 is operative to compare the stored magnitude of rollcurrent in the integrator 52 with the actual magnitude of roll currentfor the same mill stand and provide an amplified current differencetherebetween. The amplifier 62 is coupled to amplifiers 64 and 66 forenergizing the respective relays R4 and R5. The amplifiers 64 and 66have setting up elements 68 and 70 connected respectively to theirinputs.

The arrangement further comprises a sampling switch formed of anoperational amplifier 72, a relay R6 and their associated components.Specifically, a setting up element 74 including a variable capacitor anda potentiometer serially interconnected is coupled to the operationalamplifier 72 including a capacitor and a set of normally closed contactsR20, another setting up element 76 including a capacitor and a parallelcombination of a relay R7 and a potentiometer is also coupled to theoperational amplifier 72 which is, in turn, connected to an amplifier 76for energizing the relay R6. An additional setting up element 80including a variable capacitor and a resistor is connected to theamplifier 78 at its input.

The relays each includes their contacts designated by the referencecharacters denoting that relay and suffixed with a reference character0, b, c For example, the relay R1 includes two sets of normally closedcontacts R and b and three sets of normally open contacts R10, 0 and e.

On the lower portion of FIG. 6, there are shown straight line circuitsfor relays. As shown, the relay Rl winding is connected between apositive buss 8+ and a negative buss B through a set of normally opencontacts 82 and a set of normally closed contacts 84. The contacts 82are adapted to be closed when a roll current for that mill standoperatively coupled to the arrangement of FIG. 6, in this case, a firststand is to be stored while the contacts 84 are adapted to be openedwhen a roll current for the next but one stand or a third mill stand isto be stored. The relay R1 winding has connected thereacross a seriescombination of a set of normally closed contacts R80 and the relay R3winding and also a winding of a slow operating relay R9 having a timedelay for example, about 0.2 second within which a roll current for theassociated mill stand is restored to its steady-state magnitude afterthe impact drop thereof. The relay R2 winding is connected between thebusses 8+ and B through normally open contacts R90, normally closedcontact R60 and normally closed contacts R8b while a winding of a relayR8 is connected between both busses B+ and B through normally opencontact Rle, normally open contacts R40, and normally closed contactsR50 with a set of normally open contacts R8c connected across thecontacts R40 and R50;

In operation a workpiece (not shown) enters the second rolling millstand (not shown) to close the contacts 82 to energize the relays R1, R3and R9. When energized, the relay R9 closes its contact R90 thereby toenergize the relay R2. The energization of the relay R2 causes theclosure of its contacts R2c, d and e permitting the operationalamplifier 54 to compare the roll current for the first mill stand storedby the operational amplifier 52 with the actual roll current for thesame stand to produce an amplifiered current difierence between bothcurrent as will readily be understood from the previous description ofthe operational amplifiers 38 and 40 as shown in FIG. 3.

Now the relay R6 is put in its energized stated in the sampling switchto open its R60 to deenergize the relay R2. Therefore the contacts ofthe relay R2 are returned back to the original positions and theoperational amplifier 54 holds the current difference between the storedand actual roll currents therein.

On the other hand, if the output from the operational amplifier 62 iswithin a, percent for example, 3 percent of a predetermined value, thenthe relay R4 is energized and the relay R5 is deenergized. Whenenergized, the relay R4 closes its contacts R40 resulting in theenergization of the relay R8. The energized relay R8 closes its contactsR to form a self-holding circuit therefor and also opens its contactsR8a to deenergize the relay R3 to render the output from the operationalamplifier 58 null. The output from the operational amplifier 58 isapplied to the output terminal 60 leading to a speed control limiter(not shown). Also the deenergization of the relay R2 causes theoperational amplifier 72 to produce a null output thereby to deenergizethe relay R6. After the deenergization of the relay R6, a predeterminedtime interval lapses until the relay R2 is again energized. Since theoperational amplifier 72 is of the integration type, the outputtherefrom energizes the relay R6 after a predetermined time interval. Inthis way, the relay R6 is repeatedly energized at predetermined timeintervals sampling switch in the oscillation mode. Therefore thesampling switch operates as an oscillator.

Then the workpiece enters the next but one mill stand or the third stand(not shown) to open the contacts 84 whereupon the relays R1, R3 and R9are deenergized resulting in the termination of the correction of thespeed of the succeeding or second mill stand in the sampling mode ofoperation. At the same time, the process as above described in initiatedand repeated with the next but one mill stand or the third stand byusing an arrangement identical to that illustrated in FIG. 6and so on.

As computer controls have been lately developed, the direct digitalcontrol (DDC) system is increasingly substituded for the conventionalanalog control system.

For example, position controllers of the conventional construction havebeen operatively coupled to the small-sized computer operative in thetime sharing mode. Furthermore the small sized computer has been incorporated into hot strip mills for purpose of effecting the automaticstrip thickness control of the DDC type.

In FIG. 7 wherein like reference numerals designate the componentsidentical or similar to those shown in FIG. 4, there is illustrated apredicted adaptable workpiece thickness control system incorporated intoa multiple stand tandem rolling mill in accordance with the principlesof the invention. In FIG. 7 four rolling mill stands SI, SH, SIII andSIV each are shown as having a pair of horizontal work rolls and 12 andhaving the comments identical to those operatively coupled to the firstmill stand SI as shown in FIG. 4 except for the omission of themetal-in-stand circuit. Instead the load cell 26 for each mill stand isdirectly connected to a small-sized DDC computer 86 as does the currenttransformer 34, also all the speed setting up resistors 48 arecontrolled by the computer 86. Each component for one mill stand isdesignated by the same reference numeral denoting the correspondingcomponent shown in FIG. 4 and suffixed with the Roman reference numeralfor that stand. For example, the drive motor for the first stand SI isdesignated by the reference numeral 34] and the resistor for the thirdstand SIII is designated by the reference numeral 48III.

The operation of the arrangement shown in FIG. 7 will now be describedwith reference to logic flow charts as shown in FIGS. 9a, b and c. Asshown in FIG. 9a, the control operation is initiated at the start block100 in which a workpiece enters the nth stand for example the firststand SI as determined by the associated load cell 261. Then block 101interrogates to see if that workpiece is a first one to be processed inaccordance with the particular rolling reduction schedule. If so, thecontrol program goes to block 102 in which a speed correction factor Kpis obtained from a table stored in the process computer 86. If theworkpiece has been determined not to be a first one, block 103 isreached where the speed correction factor is given as a K value updatedfor the previous workpiece. Thereafter a short time delay is initiatedin block 104 until a roll current for the driving motor 241 escapes fromits impact drop and has a steady-state magnitude while the associatedmechanical stand system has been attenuated. The time delay is usuallyof about 0.2 second. After the time delay 104, the process proceeds toblock 105 where the roll current is measured repeatedly generally threetimes at different time points to determine their average 1 which is, inturn, stored in the computer. While smoothing filter means are usuallyused to eliminate the effects of a ripple current originating from theassociated pilot generator 46, a ripple voltage from the sourceresulting from the associated rectifier, etc., upon the roll current, itis required to measure the roll current repeatedly at several differenttime points and to average the measure magnitudes of roll current. Inthis case, three magnitudes of roll current have been averaged to theroll current 1 After the measurement of the roll current, the programfor the stand (n) or the first stand terminates at block 106.

Then the workpiece enters the succeeding stand (n l), in this case, thesecond stand as shown at block 200 in FIG. 9b. After the workpiece hasentered the second stand a predetermined time delay is initiated inblock 201 until the roll current for the preceding or first stand hasits steady-state magnitude. Thereafter, the roll current for the firststand is again measured three times and averaged to I in block 202. Thecontrol program then proceeds to block 203 in which Al -=1 I iscalculated and then to block 204 in which the succeeding or second standis corrected in speed according to the equation AVm/ V =K,AI, where Kpfor the first stand, this Kp may be what is stored in the table or inthe form of a prediction equation within the process computer.Alternatively it may be delivered to the DDC computer after it has beencomputed by another computer. After the speed correction thus calculatedhas been supplied to the driving system for the second stand, a timedelay 205 is initiated until that driving system has responded to thespeed correction. After that time delay the roll current for the firststand is also measured three times and the measured values of thecurrent is averaged to I, in block 206. Then in block 207 a differencebetween the roll currents previously and now measured for the firststand or A! I I, is calculated and block 207 interrogates to see whetherthe absolute value of the A! or IAI I is greater than a predeterminedfraction, for example, 3 percent of the previous current I If the l l isnot greater than 3 percent of the I then the control program goes toblock 209. This means that the operation of correcting the speed of thesecond stand has been completed and therefore that no tension is appliedto that portion of the workpiece traveling between the first and secondstands. In block 209 the roll current for the succeeding stand (n l), inthis case, the second stand is measured three times and their average Iis stored in the computer. Thus the roll current in non-tensioned statefor the next stand has been determined. The program goes to block 210 inwhich the speed correction factor Kp used to correct the speed of thesecond stand in block 204 or updated as will be described is stored inthe computer. Then the program terminates at block 21 1.

On the other hand, if the l AI 1 is greater than 3 percent as determinedby block 208 then the program is executed through a subroutine as shownin FIG. 9c. In that event the first speed correction has beeninsufficient to compensate for the particular change in roll current sothat the Kpvalue first used is considered to have been erroneous.Therefore, as shown in FIG. 90, a second speed correction is effected inblock 301. More specifically, the first value of speed correction andthe value of current correction are used to calculater a second value ofspeed correction according to the equation IIl/ II AI where V is avoltage across the motor for the stand (n l), in this case, the secondstand and AV, and AV are their changes. After a time delay 302 theprogram is executed through blocks 303, 304 and 305 similar in functionto blocks 206, 207 and 208 as shown in FIG. 9b excepting that instead ofthe I, and AI "L2 and A! are calculated as shown within blocks 303 and304 in FIG. 9c.

If the absolute value of the AI or 1 Al I is not greater than 3 percentof the I m as determined by block 305 then a new speed correction factorKp' is calculated by using the equation m/ VII in block 306. Then, inblock 307 a statistic technique is used to update the speed correctionfactor Kp according to Kp =updated wKp BKp where a+ B 1. After a timedelay 308, the program is returned back to block 209 and terminates atblock 211 as above described in conjunction with FIG. 9b.

If the block 305 answers in the affirmative then another subroutine asshown in FIG. 90 is executed. Specifically, this subroutine passes fromblock 401, through 402, 403 and 405 and to block 404. These blocks 401,through 404 are similar in function to blocks 301 through 305respectively excepting that the speed for the stand (n l) or secondstand is corrected according to the equation where AV is a change involtage across the motor for the stand (n l) or the second stand whileA1 is calculated according to AI 1 I, where I is an average of measuredroll current for the first stand, as shown within blocks 401 and 404 inFIG. 9c.

If block 405 gives a negative answer with respect to the ratio A1,; I Ithen a new speed correction factor Kp" is calculated by dividing (AV +AVV by (A1, AI in block 406 after which in the Kp is updated according toKp updated =aKp BKp' 'yKp" where a+B+y= 1 in block 407. Then the programproceeds to block 308 until it terminates at block 211 as previouslydescribed.

m/ rr= 12 On the contrary if the absolute value l A1 I is greater than 3percent of the I as determined by block 405 then block 408 interrogatesto see if time is still left for updating the stand (n 1) speed. This isbecause the execution of blocks 406, 407 and 209 must be completedbefore the head end of the workpiece enters the stand (n 2), in thiscase, the third stand. If block 408 answers in the negative the programproceeds to block 209 with a time delay 308. Otherwise the program isreturned back to block 302 and proceeds through the successive blocks303, 304

In summary, the roll current for the stand (n) wherein may be equal to lis stored in the computer and a variation in roll current for the stand(n) is measured after the workpiece has entered the stand (n I). Thenthis variation in roll current for the stand (n) is utilized todetermine a force exerted on that portion of the workpiece travelingbetween the stand (n) and the stand (n 1). That is, the speed correctionfor the stand (n l is predicted from the variation in roll current forthe stand (n) thereby to update the speed of the stand (n l).Thereafter, a variation in stand (n) current is again measured therebyto correct the prediction equation for speed correction and to updatethe stand (n l) speed. If the variation in roll current is not withinpredetermined limits the routine as above described is repeated tofurther update the stand (n l) speed until the variation in stand (n)current is put within the predetermined limits. Thereafter the rollcurrent for the stand (n l) is measured and stored in the computer as areference value utilized when the workpiece will have entered a stand (n2) or a third stand. Also the last speed correction factor updated isstored in the computer and utilized to process the succeeding piece. Theroll currents for the respective stands change as the functions of timeas shown in FIG.

8. In FIG. 8, the axis of abscissas represent time and the axis ofordinates represents a roll current 1 for the stand (n) or the firststand SI in FIG. 8b and a roll current I, for the stand (n l) or thesecond stand 81! in FIG. 8cand aspeedN ofthe stand (n+ l) in FIG. 8d. Asshown in FIG. 8b the roll current for the stand SI is rised in responseto the entering of a workpiece into that stand and has a steady-statemagnitude at and after a time point to. At that time point, the rollcurrent I, is stored in the computer and provides a roll current Istored in non-tensioned state at a time point t after the workpiece hasentered the second stand SII the roll current I, is measured. Themeasured current I t is less than the tension-free current I, stored bya magnitude of Al indicating that a tension is exerted on that portionof the workpiece traveling between both stands SI and 811. Therefore thespeed N of the second stand SII is caused to decrease as shown in FIG.8d. This causes a decrease in that tension permit the roll current I,for the first stand SI to approach the steady-state magnitude 1, stored.Similarly, the speed of the second stand SH are updated at time pointst,, i to progressively decrease difference between the steadystatecurrent I, stored and the measured currents for the first stand as shownat Al Al in FIG. 8b so that the roll current I, approximates thesteady-state current I, stored. Eventually, the roll current I is equalto the steady-state current I, stored whereupon the non-tensioned stateis established between the first and second states SI and SI] with thespeed correction factor Kp provided as a function of the magnetic fluxin the associated driving motor. As well known, parameters of rollingstatus such as a roll reaction P, a torque G, a forward rate f, and abackward rate e have certain mathematical relationship with workpieceparameters such as an entry workpiece thickness H, a delivery workpiecethickness h, an entry workpiece width W, a delivery workpiece width W, abackground tension t, a forward tension t,, a temperature T, acoefficient of friction p. etc. Therefore their models may be used tocompute the speed correction factor Kp.

From the foregoing it will be appreciated that the invention hasprovided a workpiece thickness control system for a multiple standtandem rolling mill capable of continuously rolling workpieces whileeach pair of adjacent rolling mill stands do not exert any tension orany pressure on that portion of the workpiece traveling therebetween.

What is claimed is:

1. In a workpiece thickness control system for use with a multiple standtandem rolling mill comprising at least two rolling mill stands, thecombination of means for measuring a roll current for a first one of therolling mill stands, means for determining a variation in roll currentfor the first rolling mill stand, means for predicting a speedcorrection for a second rolling mill stand from the variation in rollcurrent for the first rolling mill stand to modify the speed of thesecond rolling mill stand, and means for controlling the speed .of thesecond rolling mill stand so as to put a variation in roll current againmeasured for the first rolling mill stand within predetermined limits.

2. In a workpiece thickness control system for use with a multiple standtandem rolling mill comprising at least two rolling mill stands, thecombination of means for controlling said oscillator forming saidsampling switch to put said current difference within predeterminedlimits, thereby to adjust the speed of the second rolling mill stand.

1. In a workpiece thickness control system for use with a multiple standtandem rolling mill comprising at least two rolling mill stands, thecombination of means for measuring a roll current for a first one of therolling mill stands, means for determining a variation in roll currentfor the first rolling mill stand, means for predicting a speedcorrection for a second rolling mill stand from the variation in rollcurrent for the first rolling mill stand to modify the speed of thesecond rolling mill stand, and means for controlling the speed of thesecond rolling mill stand so as to put a variation in roll current againmeasured for the first rolling mill stand within predetermined limits.2. In a workpiece thickness control system for use with a multiple standtandem rolling mill comprising at least two rolling mill stands, thecombination of means for storing a roll current for a first rolling millstand, means for determining a current differencE between the storedroll current for the first rolling mill stand and a roll current for asecond rolling mill stand measured after a workpiece enters the secondrolling mill stand, a sampling switch comprising an oscillator, andmeans for controlling said oscillator forming said sampling switch toput said current difference within predetermined limits, thereby toadjust the speed of the second rolling mill stand.